People keep asking me which sizes I use and what I recommend.
That is a question I cannot really answer.
Everyone is different, has different experience, different weight, different conditions.
So wind ranges are relative, since e.g. I can hold my 18 in gusts up to 25 knots in warm air...in colder air it is less, since the air is more dense.
In general I recommend as big as you can handle it. Big, because it gives you more hangtime and the kite will have slower turning, which means more control.
For example, I use my 18 sqm kite 95% of the time, and I use the inner bar connection, so it is as slow as possible.
But...too slow is also not good...you still need to be able to loop the kite for tricks like Jesus Walks.
When you buy a kite, best is to try it before, and see the control, bar feedback, hangtime and lift of the kite. Hangtime is very important, because you need time in the air for quiet many tricks.
Then start in lighter winds, and keep experiencing it with stronger and stronger winds, to see where your limit is.
At the same time use a smaller board to be able to edge and control the power of the kite.
Use a depower as long as possible, this way you can also hold more pressure.
Back then everyone said, the C-kites don't have much depower...but the people used shorter depowers, leading to less depower possibility of the kite. I always had a long depower, so I always had a bigger range on C-kites as well.
When using bigger kites, you should also be more aware regarding safety and weather.
Launching and landing kites need more precaution. Good explanations with video you will find on my Airstyle DVD: www.airstyle.tv
I hope it helps!